天然气水合物是现今最具开发价值的非常规能源，但其一般沉积于深海底层或永久冻土层，开采难度高，且此类开采活动往往是海底滑坡等地质灾害的诱因，因此对天然气水合物水下开采的模拟也成为国内外关注的热点，研究此类问题有重要的社会价值。天然气水合物开采及其诱发的海底滑坡现象，本质上可以视为一种自然界中的流固耦合现象。针对这一现象，本文拟采用CFD-DEM方法来开展模拟研究。CFD-DEM方法就是利用计算流体力学（CFD）去模拟区域内流体运动，用离散元方法（DEM）去模拟区域内颗粒受力和运动的一种耦合方法，它以纳维-斯托克斯方程为流体相的控制方程，以牛顿第二定律为固体相的控制方程，通过传递颗粒与流体间的相互作用力（如拖曳力）实现流体与颗粒运动的耦合。该方法通过耦合软件CFDEM实现计算。CFDEM可以调用CFD开源软件OpenFOAM和DEM库liggghts分别计算流体和颗粒运动，并在两者间传递实现耦合计算的动量交换。在软件原本的功能中，cfdemPiso这一计算程序仅能计算单相流体与颗粒的相互作用，计算区域内不能出现多相流体。本文参考OpenFOAM中已有的多相流计算程序interFoam，对现有的程序进行改进，实现了多相颗粒流的计算，可以用于模拟颗粒引起的界面变化。考虑计算模块的正确性，本文比照前人文献的结果进行了模拟计算，从流体流场分布、固相颗粒速度等角度进行了分析，发现与文献结果具备一致性，这也能说明计算软件的可靠。考虑天然气水合物一般以夹层的形式出现在水下岩土体内，本文建立的岩土体模型也参考了这一点，均包含小颗粒夹层，用于模拟在岩土体内易滑动的天然气水合物夹层。本文的模拟首先利用CFDEM中颗粒自由沉降获得密集分布的颗粒，在通过坐标筛选得到参与建立模型的颗粒，利用CFDEM模拟不同坡角的颗粒堆积体的滑移，最终获得的与时间相关的结果利用paraview软件进行处理分析。不同坡角颗粒体的模拟，目的是获取稳定坡形的临界坡角值，同时模拟出不同运动速度的坡体对水体和水面的扰动效果。稳定的坡形，可以作为下一步天然气水合物开挖的起始坡形，用于探索开挖扰动，最终获取一个辅助预测的安全参数。
Gas hydrate currently becomes the most valuable unconventional energy source for development, but it is generally deposited on deep seabed or permafrost, which is difficult to mine, and such mining activities are often the cause of geological disasters such as submarine landslides. Therefore, the simulation of gas hydrates’ underwater mining has also become a hot spot all over the world, and has important practical significance.The mining of gas hydrate and the phenomenon of submarine landslides induced can be regarded as a fluid-solid coupling phenomenon in nature. In response to it, this article intends to use the CFD-DEM method to carry out simulation research. The CFD-DEM method is a coupling method, using the computational fluid dynamics (CFD) to simulate the fluid movement in the area, and the discrete element method (DEM) to simulate the force and movement of particles in the area. The Navier-Stokes equation is the governing equation of the fluid phase, and Newton's second law is the governing equation of the solid phase. The coupling of fluid and particle motion is realized by transmitting the interaction force between particles and fluid (such as drag force).This method realizes the calculation through the coupling software CFDEM. CFDEM can use the CFD open source software OpenFOAM and DEM library LIGGGHTS to calculate the fluid and particle motions respectively, and transfer the momentum between the two to realize the momentum exchange of the coupling calculation. In the original function of the software, the calculation program cfdemPiso can only calculate the interaction between single-phase fluid and particles, and multiphase fluid cannot appear in the calculation area. Referring to the existing multiphase flow calculation program interFoam in OpenFOAM, this article improves the existing program to realize the calculation of multiphase fluid-particle flow, which can be used to simulate the change of interface caused by particles.Considering the correctness of the calculation module, this article carries out a simulation based on the results of previous literatures. By analyzing the fluid field distribution and the solid particle velocity, it finds that the results are consistent with the literature results, which can also explain the reliability of calculation software. Considering that gas hydrates generally appear in the form of interlayers in underwater rock –soil mass, the models established in this article also refer to this point. They all contain small particle interlayers to simulate the easy sliding of gas hydrate interlayers in the rock-soil mass. In this paper, through using the free sedimentation of particles in CFDEM, densely distributed particles are obtained firstly. Then particles participating in the establishment of the model are obtained through coordinate screening. Using CFDEM to simulate the slippage of particle accumulations with different slope angles, time-related results can be obtained and analyzed by the processing software ParaView. The purpose of simulating particle accumulations with different slope angles is to obtain the critical slope angle for stable slope, and to simulate the disturbance effect of slope bodies with different moving speeds on the water body and surface. The stable slope can be used as the starting slope for the next step of gas hydrate excavation, used to explore disturbances of excavation. Finally a safety parameters for auxiliary prediction is obtained.
